Electrostatic printing



Dec. 6, 1960 H. G. GREIG 2,963,365

ELEcTRosTA'rIc PRINTING Filed Feb. 16, 1956 2 Sheets-She'et 1 INVENTOR. Hanau: EREY EREIE /TTRNEY 2 Sheets-Sheet 2 Filed Feb. 16, 1956 INVENTOR. Hamam GREY Emana @ifm/Ey ELECTROSTATIC PRINTING Harold Grey Greig, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Filed Feb. 16, 1956, Ser. No. 565,985

14 Claims. (Cl. 96--1) This invention relates to electrostatic printing and particularly, but not necessarily exclusively, to improved electrophotographic recording elements and to improved electrostatic printing processes utilizing said improved recording elements. y

An electrostatic printing process is that type ofpro'c'ess for producing a visible record, reproduction orV copy which includes as an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern on an electrically-insulating layer. The process may also include the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original except that it may be different in size, color, or contrast value.

Previous electrostatic printing processes usually produce visible images on only one side of a recording element. Where visible images are produced on both sides of the recording element, previous processes comprise two successive and identical sequences of steps; first producing a visible image on one Side of the sheet, then producing a visible image on the other sideof the recording element. Descriptions of typical previous processes may be found in UlS.. Patent No. 2,297,691 to C. F. Carlson and in C. I. Young and H. G. Greig, Electrofax Direct Electrophotographic Printing on Paper, RCA Review, vol. XV, No. 4, December 1954.

An object of this invention is to provide improved electrostatic printing processes.

Another object is to provide improved electrostatic printing processes for simultaneously producing different latent electrostatic images on opposite sides of a single recording element, and improved processes for simultaneously producing different visible images on opposite sides of a singleV recording element.

A further object is to provide improved electrophotographic recording elements. Y

Another object is to provide improved electrophotographic recording elements capable of having different latent electrostatic images simultaneously produced on opposite sides thereof.

The improved electrophotographic elements of the invention have two opposed surfaces, one of said surfaces comprising a first photoconductinglayer capable of being electrostatically-charged in one polarity of charge and the other of said surfaces comprising a second photoconducting layer capable of being electrostatically-charged in the opposite polarity of charge. Such photoconducting layers may be mounted separately or may comprise an integral unit, such as a single `paper backing between and supporting both layers. The second photoconducting layer may have the same or fa different range o f spectral sensitivity as the first photoconducting layer. The backing or support for one ofthe photoconducting layers may be capable of transmitting light to which the other photoconducting layer is sensitive.

The improved electrostatic printing processes of the invention comprise providing a recording element of the invention, producing a blanket electrostatic charge of one nit-edf States Patent polarity on the surface of the first photoconducting layer, and simultaneously producing a blanket electrostatic charge in the opposite polarity of charge on the surface of the second photoconducting layer, exposing the charged first photoconducting layer to a first light image within its range of spectral sensitivity thereby producing a rst latent electrostatic image in the first photoconducting layer, exposing the charged second photoconducting layer to a second light image within its range of spectral sensitivity thereby producing a second latent electrostatic image in the second photoconducting layer. The first and second light images may be projected from opposite sides of the recording element either simultaneously or successively. Where the recording element so allows, the light images may be projected from the same side of the recording element either simultaneously or successively. The two latent electrostatic images so produced may be developed to visible images by the simultaneous or successive application of finely-divided developer substances to the electrostatic images.

The foregoing objects and other advantages are more fully described in the following detailed description when read in conjunction with the accompanying drawings of which:

Figure l is a partially-schematic, sectional, elevational View of an apparatus for simultaneously charging opposite surfaces of an improved recording element of the invention in opposite polarities of charge,

Figure 2 is a partially-schematic, sectional, elevational view of an arrangement for producing `a first light image on the first photoconducting layer of the recording element of Figure 1,

Figure 3 is a partially-schematic, sectional, elevational View of an arrangement for producing a second light image on the second photoconducting layer of the recording element of Figure 1 Figures 4(11), 4(b), 5,y 6, 7(51), and 7(1)) are partiallyschematic, sectional, elevational views of alternative arrangements for exposing the charged surfaces of the irnproved electrophotographic recording elements of the invention,

Figures 8(a), 8(b), and 9 are partially-schematic, sectional, elevational views of alternative arrangements for developing visible images from latent electrostatic images upon the photoconducting surfaces of the electrophotographic recording elements of the invention, and

Figure 10 is a partially-schematic, partially-sectional, elevational view of an apparatus for Continuous printing utilizing an improvedv electrophotographc recording element and an improved method according to the invention.

Similar reference characters are applied to similar components throughout the drawing.

Example 1 The following example is given for the purpose of illustrating an improved recording element and an improved process of electrostatic printing according to the invention. A

Referring to Figure 1, a' sheet 21 of paper has a coating on one surface thereof comprising a first photoconducting layer 23 comprising a panchromatically sensitive zinc oxide dispersed in a silicone resinv The opposite surface of the sheet 2l has a second photoconducting layer 2S thereon comprising a white zinc oxide dispersed in a-silicone resin. The steps for preparing this recording element will be `more yfully described below.

The recording element is passed slowly. one or more t times, between la parof opposed corona discharge units.

Patented Dec. 6, 1960 speci: to ground. The second array of wires 43 faces the second photoconducting layer 25 and is spaced therefrom by about 0.5 inch. The second array of wires 43 is maintained at a potential of about -6200 volts with respect to ground. In passing between the wires 41 and 43, a positive electrostatic charge forms on the first photoconducting layer 23 and simultaneously a negative electrostatic charge forms on the second photoconducting layer 25.

Referring to Figure 2, the first photoconducting layer 23 is exposed to a first light image. This is accomplished by contact printing through a negative photographic film master 27 placed on the second photoconducting surface 25, and exposing to the light from two 15 watt gold fiuorescent lamps 31 held about 20 inches away for a period of about 3 seconds. The blanket negative electrostatic charge on the second photoconducting layer 25 is not appreciably affected by this light during exposure. The second photoconducting layer 25 and the paper backing 21, however, transmit the light from the gold fluorescent lamps 31. The first photoconducting layer 23 is sensitive to light from the gold fluorescent liamps 31. Thus, when the first light image is incident upon the first photoconducting layer 23, a first latent electrostatic image is formed on the first photoconducting layer 23 by selective discharge of the areas of the first photoconducting layer 23.

Referring to Figure 3, the second photoconducting layer 25 is exposed to a second light image. The negative photographic film master 27 is replaced with a positive photographic film master 29 and is exposed to light from two 4 watt ultraviolet (Sylvania Backlight) lamps 33, held about 20 inches away for about 1/2 second, forming a second latent electrostatic image in the second photoconducting layer 25. Both photoconducting layers 23 and 25 are sensitive to light from this source, but the second photoconducting layer 25 is a good filter to the ultraviolet light from the ultraviolet lamps 33. The exposure necessary to form the second latent electrostatic image on the second photoconducting layer 25 has little or no effect on the first latent image in the first photoconducting layer 23.

Referring to Figures 8(11) and 8(b), the latent electrostatic images formed by the foregoing steps are now developed to visible powder images having substantially the same configuration as the respective latent electrostatic images. The same developer mix is applied to each photoconducting layer 23 and 25. For this purpose a magnetic brush 35 comprising a mixture of iron filings and a finely-divided developer powder held in a magnetic field is contacted across the surface of the second photoconducting layer 25 depositing developer powder 39 in substantial configuration with the second latent electrostatic image. Then the magnetic brush 35 is oontacted across the first photoconducting layer, depositing developer powder 39 in a configuration substantially corresponding to the second latent'electrostatic image. The respective powder images 37 and 39 so produced are then fixed in situ as by spraying with an adhesive or by heating if the developer powder is thermoplastic. The recording element is preferably maintained in darkness during the charging, exposing and developing steps.

According to the example, opposite surfaces of the improved electrophotographic recording element are simultaneously charged. Two different latent electrostatic images are then produced on the oppositesurfaces, which latent electrostatic images are then developed to visible powder images.

The recording element 4 pable of being electrostatically-charged in the opposite polarity of charge. Since many photoconducting layers are chargeable in only one polarity of charge, the materials for the respective layers must be carefully selected in order to satisfy the basic requirements of the improved recording elements of the invention.

The photoconducting layers of the recording elements of the invention preferably comprise a particulate photoconductor dispersed in an electrically-insulating, filmforming vehicle. However, other photoconducting layers may be used. In order to determine whether or not a particular photoconductor is suitable in the present invention, a test of its photoconducting properties may be made as follows. A small quantity of the powdered substance is compressed under high pressure, i.e., about 15,000 lbs. per square inch to form a pellet. Electrodes, as of silver paste, are applied on a surface of the pellet leaving a square area of surface uncoated. The pellet is placed in a monochromator with the uncoated surface area facing the light source, and exposed to successive wave lengths of light throughout the spectrum. During the exposure, D.C. voltage is placed across the electrodes and the current flowing between the electrodes is measured as a function of wave length, with the intensity of radiation maintained constant.

The photoconductors which are suitable in the invention are those which are substantially non-conductive in the dark and which exhibit a surface photoconductivity above a certain level when illuminated. In testing photoconductors to determine their suitability in the invention, it is convenient to express the results of the measurement of the test as surface photoconductivity because substantially lall of the light is absorbed within a thin layer at the surface of the pellet. It has been found that, to be useful in the present invention, the substance selected should have a surface photoconductivity of at least about 10-9 ohms-l/square/watt/cm-2. Having established the threshold value of photoconductivity for the purposes of the invention, it is possible to test other photoconductors otherwise suitable from the standpoint of compatibility, dark resistivity, etc. in order to determine whether they can be used from the standpoint of photoconductivity.

The electrically-insulating, hlm-forming vehicle may be any one of a number of substances. The vehicle should preferably have a relatively high dielectric strength. Examples of suitable resins are polyvinyl acetate, copolymers of vinyl acetate, polystyrene and silicone resins. Other resin-like materials such as cellulose esters and cellulose ethers, for example methyl cellulose, ethyl cellulose, ethyl nitrate, may be used. Natural resins such as parafiin Iand carnauba wax may also be used.

The usual plasticizers for the vehicles mentioned above may be used to increase the flexibility of the coating provided the plasticizer does not impair the insulating properties of the final photoconducting coating.

The following are examples of coating compositions which produce photoconducting layers which may be charged positively.

Example 2.-Intimately mix grams of a 60% solution of silicone resin in xylene (a commercially available solution of this type is G.E. SR-82 marketed by the General Electric Company, Silicone Products Division, Waterford, N.Y.); 106 grams of toluene; and 120 grams of panchromatically-sensitive zinc oxide. The panchromatically-sensitive zinc oxide may be prepared by heating a mixture of 2 parts ammonium carbamate acid carbonate (commercially available ammonium carbamate) 'and about one part of white zinc oxide. The white zinc oxide has a sensitivity peaked in the ultra-violet region of the spectrum; with comparatively lit-tle sensitivity in the visible region of the spectrum. The panchromatically-sensitive form of vZinc oxide is sensitive over the entire visible spectrum,

ceases Example '3.-lnti'mately mix :80.grams of a.60% solution 4of silicone resin .in xylene, such :as thesilicone .,resin of ExampleZ'; with :1'06 ,grams of :to1uene;.and 1.20:g`rams of fiuorescent `cadmium sulfide, such as Flourescent 2220 marketed by the New Jersey Zinc Company, lPalmerton, Pa.

Example 4.-Intimateiy mix 80 grams ofa 60% so'lution of silicone resin'in xylene, such as the silicone Vresin of Example 2; with 120 grams of a vluminescent Silveractivated zinc sulfide, such as RCA No. 33-'Z256markete'd by the Radio Corporation of America, Tube Division, Harrison, New Jersey and 106 grams of toluene.

The following are examples of coating compositions which produce photoconducting coatings 1which-may -be charged negatively. i

Example 5.-Intima=`tely`.mix 120 grams of white zinc oxide, such as Florence ,Green Seal 8 marketed bythe New Jersey Zinc Company, Palmerton, Pennsylvania; `106 lgrams of toluene; and 80 grams of a silicone resin containing about 60% solids, such as the resin of Example 2,.

Example 6.--The mixture of Example 2 produces v-photoconducting coatings .which may also be charged negatively.

Example 7..-'=l`he mixture of `lExample 3produces lphotoconducting coatings which may also be charged negatively.

Example 8.-'Ihe mixture of Example @produces photoconductng layers which may also be charged negatively. Y

To prepare an electrophotograpliic coating, `one o f the foregoing mixtures, say the mixture of `Example 2, is blended for about an 'hour in a porcelain mill to obtain uniformity. Ordinary white bond paper is then coated on one surface thereof, draining off the excess composition and thenair drying the coating. The mixture may also y'be applied by thinning with acetone and spray coating or by anyother ofthe usual paper coating techniques such as now coating, roller coating and thelike.

The photoconducting layers of the invention yare most easily prepared by coating a suitablebacking with a kcoating mixture in any one of the coating methods well known in the coating art. One method is to dissolve Vthe vehicle in a solvent capable of effecting solution, mixing in the particulate photoconductor, coating ont-he backing, and then drying. Alternatively, the particulate photoconductor may be dry-blended by kneading with the vehicle heated `to a sufcientlyhigh temperature to render the vehicle plastic and below the decomposition temperature of the photoconductor, coating on a suitable backing and then cooling.

The support or backing for coating -the photoconducting layers may be either a relatively good electrical conductor, such as aluminum foil or carbon loaded paper, or a relatively poor conductor such as ordinary paper or a ceramic plate. The backing may be an opaque material or a transparent material such as glass or cellophane. The backing may be coated or, if adapted l-to impregnatiommay beimpregnated with the particular compositions.

The recording element of the invention may comprise any combination of two photoconducting layers one of which is capable of being positively charged and the other of which is capable of being negatively charged. The following are examples of various combinations: (l) a paper sheet ,coated on both sides with the mixture of Example 2; (2) 4va paper sheet coated on one side with the mixture of Example 2 and on the other side with the mixture of Example 5; (3) aluminum foil coated on one side with the mixture of Example 4 and on the other side with the mixture `of Example 6; (4) a cellophane sheetcoated on one side with the mixture of Example 3 and on the other side with :the mixture of Example 8; (5) two selfsupporting photoconducting layers may be placed against one another; (t6) vpaper sheets having only one photoconductive layer thereon may ,be placed `together with the photoconductive layers facing out. Thus, the photoconfductingl-ayers are :selected for the :usual characteristics ,Soif rangefcf spectral-sensitivity, -decay characteristics, storage properties and also for the ability to be electrov'statically charged to opposite polaritie's.

Charging .Referring again to Figure 1, -the recording elements of the invention are simultaneously charged on both surfaces in opposite polarities by passing the recording elements between a pair of opposedfcorona apparatus maintained at opposite potentials. The photoconducting layer capable of being positivelycharged should face the posi- -tive corona appara-tus and the photoconducting layer capable of being negatively charged should face theenegative corona apparatus. Alternatively one or both of the corona apparatuses may bereplaced with a solid ior a liq- -uid electrode which gives good contact over the surface `ofthe photoconducting layer to ywhich it is contacted. It is also `possible to replace one or both of the corona apparatuses with suitable rubbing means to charge the pho- .toconducting layer by friction.

Exposure Exposure is `accomplished byproducing upon eachpho- .toconducting layer an incident image of light within the yrange of spectral sensitivityof the particular layer. Such a light image may be produced by the techniques of contact printing as shown inFigures 2 and 3, or by projec- Vtion las Ashown .in Figures 4 to 7. Wherever light strikes the-photoconducting layer, the electrostatic charge thereon is reduced or removed. This leaves an electrostatic image or pattern ,of charges corresponding-to the non-illuminated .portions of the incident light image.

The ,incident light image may originate from opposite sides lof the ,recordingelement as shown in Figures 2l(a), 4(b) and Figure 5. In these arrangements, the photoconducting layers and the backing may be transparent or opaque to light. Where the backing is transparent, light incidentupon one photoconducting layeris filtered-by the 'layer and vdoes not penetrate to disturb the electrostatic charge or image `on the other .photoconducting layer. The incident light images may originate from "the `same side 0f the recording element, if the backing or support and the photoconduct-ing layer on vthe side from which the images originate transmit light to which the other photoconducting layer is sensitive. Thisis shown in Figures 6, 7(a) and 7(1)). The two light images may be 'incident upon the respective photoconductingflayers simultaneously as shown in Figures 5 and 6, or successively as shown in,Figures 4(a) and .4(b), and 7(a) and 7(b).

Development The electrostatic `images may be -stored for a time if desired. Ordinarily, the next step is to -develop the velec- `trostatic images with a finely-divided developer substance such as a finely-divided powder or an ink mist. Referring to Figures 8(a) and 8(b), development of 'the electrostatic image is accomplished by maintaining the recording element in darkness ,and passing a developer brush `35 containing a developer powder across the surface of the rst photoconducting layer 23 bearing the first latent electrostatic image and across the second photoconducting layer 25 bearing Vthe second latent electrostatic image. The first and second surfaces may be contacted in any order. Areas of developer powder are deposited in substantial configuration with the respective lightimages.

The developer `brush comprises mixture of magnetic carrier particles, for example, powdered iron and `the developer powder. The mixture is secured in a magnetic field by a magnet 57 to form a developer brush.

A preferred carrier particle for the developer brush consists of alcoholized iron, i.e., iron particles free from grease and other impurities soluble in alcohol. These iron particles are preferably relatively small in `size being .in Vtheir largest dimension between about 0.002

A7 to 0.008 inch. Satisfactory results are obtained with a somewhat wider range of size between 0.001 and 0.020 inch.

The preferred developer powder may be prepared as follows. A mixture comprising 200 grams of 200 mesh Piccolastic Resin No. 4358 (an elastic thermoplastic resin composed of polymers of styrenes, substituted sty- -rene and its homologs) marketed by the Pennsylvania Industrial Chemical Company, Clairton, Pennsylvania; 12 grams of carbon black G marketed by the Eimer and Amend Company, New York, New York; l2 grams of spirit nigrosine SSB marketed by the Allied Chemical and Dye Company, New York; and 8 grams of Iosol Black, marketed by the Allied Chemical and Dye Company, are thoroughly mixed in a stainless steel beaker at about 200 C. The mixing and heating should be vdone in as short a time as possible. The melt is poured into a brass tray and allowed to cool and harden. The hardened mix is then broken up and ball milled for about 20 hours. The powder is screened through a 200 mesh screen and is then ready for use as a developer powder. This powder takes on a positive electrostatic charge when mixed with glass beads or iron powder. It develops an electrostatic image composed of negative charges. Two to four grams of developer powder and one to 100 grams of magnetic carrier particles are blended together to give the completed developer mix.

The developer powder may be chosen from a large class of materials. The developer powder is preferably electrostatically-charged to aid in the development of the electrostatic images. The powder may be electrostatically-charged because the powder (l) is electroscopic, or (2) has interacted with other particles with which it is triboelectrically-active, or (3) has been charged from an electric source such as a corona discharge. Examples of suitable developer powders are powdered zinc, powdered copper, carbon, sulfur, natural and synthetic resins or mixtures thereof.

The developer powder may be applied to the latent electrostatic images in other ways. For example, it may be dusted on or it may be mixed with glass beads or other suitable gravity carrier particles and then bringing the mixture formed thereby into contact with the photoconducting layer bearing the latent electrostatic image to be developed. The beads serve merely as a temporary carrier releasing the developer powder particles upon contact with oppositely charged areas of the photoconducting layer.

The latent electrostatic images may be developed successively as shown in Figures 8(a) and 8(b), or may be developed simultaneously as shown in Figure 9.

The developed image is now fixed to the photoconducting layer upon which it resides. If the developer powder or the vehicle of the photoconducting coating has a relatively low melting point, the visible image may be fixed by heating, for example, with an infrared lamp, to soften either the photoconducting layer or the developer powder particles to cause them to adhere to one another. Alternatively, the powder image may be pressed into the photoconducting layer. Another method of fixing the powder image is to apply a thin coating of a solvent for the material of the powder image. The solvent may soften the developer powder particles and cause them to adhere both to one another and to the photoconducting layer on which they reside. Alternatively, a solvent may be used to soften the photoconducting layer and cause the developer powder particles to iadhere thereto. Upon standing and preferably with the application of a slight amount of heat, the solvent is evaporated from the photoconducting layer.

Referring to Figure 10, the improved recording element may be used and the improved methods of the invention may be embodied in an improved continuous electrostatic printing process and apparatus. A continuous -web comprising a paper substrate 21 having on one surface thereof a photoconducting layer 23 capable of being positively charged, for example, the photoconducting layer produced with the mixture of Example 2 and on the opposite surface thereof a photoconducting layer 25 capable of being negatively charged such as the layer produced from the mixture of Example 5 is passed between idler roller 47 and takeup rollers 49 and 50. The continuous web first passes a station where a substantially uniform positive electrostatic charge is deposited on the first photoconducting layer 23 and a uniform negative electrostatic charge is produced on the second photoconducting layer 25. Such charges are produced from two arrays of corona discharge wires 55 and 57 on opposite sides of the recording element and connected to voltage sources which maintain the first array 55 facing the first photoconducting layer 23 at a positive polarity of +6400 volts and the second array facing the second photoconducting layer 25 at a potential of -6400 volts.

The continuous web passes to a station where an image of light is projected from a projector 59 incident upon the rst photoconducting layer 23. For this purpose, yellow, orange or red light may be used, for eX- ample, yellow light. The negative electrostatic charge on the second photoconducting layer is not aected by this light. However, the first photoconducting layer is sensitive to this light and the charge in the illuminated areas of the first photoconducting layer 23 is discharged leaving a first latent electrostatic image in substantial configuration with the rst light image. A second image of light to which the second photoconducting layer is sensitive is projected from a projector 60 on the second photoconducting layer. The illuminated areas of the second photoconducting layer` 25 are discharged leaving a second latent electrostatic image in substantial configuration with the second light image. The second photoconducting layer 25 prevents the light of the second light image from penetrating to the first photoconducting layer 25 and has little or no effect on the irst latent electrostatic image previously formed on the first photoconducting layer 23.

The continuous web advances to a station where the latent electrostatic images residing on the photoconducting layers are developed to visible powder images. For this purpose, a pair of developer means located on opposite sides of the continuous web bring the magnetic developer mix previously described into contact with the photoconducting layers as the layers pass the station. Each developing means comprises a grounded rotary pole piece and 75 of a magnetic structure with spaced parallel opposed elliptical discs 77 and 77 facing the photoconducting layer to be developed. A magnetic eld is maintained in the gap between the opposed elliptical discs 77 and 77', the rotary magnetic pieces 75 and 75 and through a stationary magnetic piece (not shown) magnetically connecting the rotary magnetic pieces 75 and 75. A reservoir 81 holds quantities of developer mix in contact with the discs 77 and 77. The rotary pole pieces 75 and 75 rotate the discs 77 in opposite directions such that the developer mix forms on the periphery on the disc in brush-like filaments 81 and 81' and is swept upwards and across on the surfaces of the photoconductive layers 23 and 25 in the direction in which the photoconducting layers 23 and 25 pass the station. Developer powder particles deposit upon sef lected areas producing visible powder images 83 and 85 on the respective photoconducting layer.

The continuous web bearing the visible powder images 83 and 85 now passes to a station where the visible images are fixed to the photoconducting layers. For this purpose a pair of radiant heaters, each comprising resistance wires 87 and 87' connected to a voltage source is maintained in closely spaced relation with each of the visible powder images. Heat radiated from the wires soften the thermoplastic resin of the developer powder 9 comprising the particles thereof to adhere to the `phot/ oconducting layer on which it resides producing `fixed visible images 89 and 91. `I'he continuous web bearing the fixed visible image now passes between takeup rollers 49 and 50 where it may be rolled up on a roll or cut into convenient lengths. Y n

There have been described improved electrophotographic recording elements comprising two photoconducting layers each of which is chargeable in opposite polarities. There have also been described improved methods of electrostatic printing wherein latent electrostatic images may be produced simultaneously on opposite sides of the recording elements of the invention, and may be developed and fixed simultaneously.

What is claimed is: n

l. In a method of electrostatic printing, the steps comprising: providing a recording .element having two opposed layers, each of which has an exposed outer surface, one of said layers comprising a first photoconducting insulating layer sensitive to and absorptive of light in a first spectral range and consisting essentially of a first finely-divided photoconductor dispersed in an electrically-insulating film-forming resinous vehicle and having the capability of being electrostatically-charged in a first polarity of charge, the other of said layers comprising a second photoconducting insulating layer sensitive to and absorptive of light in a second spectral range at least a portion of which is different from said first spectral range, said second layer consisting essentially of a second finely-divided photoconductor dispersed in an electrically-insulating film-forming resinous vehicle and having the capability o-f being electrostatically charged in a second polarity of charge; producing a blanket electrostatic charge in said first polarity on the exposed surface of said first photoconducting layer and producing a blanket electrostatic charge in said second polarity on the exposed surface of said second photoconducting layer by simultaneously subjecting each of said layers to a corona discharge of appropriate polarity; exposing said charged second photoconducting layer to a first light image having a spectral range within said second spectral range but different from and not included in said first spectral range, thereby producing an electrostatic image on said second photoconducting layer while leaving said electrostatic charge on said first photoconducting layer substantially undisturbed, exposing the outer surface of said charged first photoconducting layer to another llight image within said first spectral range, thereby producing a second electrostatic image on said first photoconducting layer while leaving the electrostatic image on said second photoconducting layer substantially unaffected.

2. A method according to claim l including developing said first and second electrostatic images with a finely-divided developer substance to produce visible images in substantial configuration with said electrostatic images on each of said surfaces respectively.

3. A method according to claim l wherein said first and second light images are projected simultaneously.

4. A method according to claim l wherein said first and second light images are projected successively.

5. A method according to claim l wherein said second photoconductor comprises panchromatically-sensitive zinc oxide and said first photoconductor comprises white zinc oxide.

6. A method according to claim l wherein said first and second photoconducting layers comprise a lsingle integral record element,

f7. A method according 4to claim l wherein Said first and second photoconducting layers comprise separable parts, with respect to one another.

8. A method of electrostatic printing comprising providing a paper sheet coated on one surface with a first mixture comprising photoconducting white zinc oxide dispersed in an lelectrically-insulating silicone resin, said papersheet having its opposite surface coated with a second mixture comprising photo-conducting panchromatically-sensitive zinc oxide dispersed in an `electricallyinsulating silicone resin, producing a blanket negative electrostatic charge upon the surface of the coating of said first mixture and a blanket positive electrostatic charge on the surface of the coating of said second mixture by simultaneously subjecting veach of said surfaces to a corona discharge of appropriate polarity, projecting an image of light including wavelengths shorter than 4200 A. and substantially free of light longer than 4200 A. upon the `coating of said first mixture, projecting an image of light including wavelengths longer than 4200 A. and substantially free of Vlight shorter than 4200 A. upon the coating of said second mixture, and then developing the electrostatic images produced in each of said coatings respectively.

9. A recording element for electrostatic printing comprising a substantially translucent cellulosic backing sheet, a first photoconducting insulating coating comprising photoconductive white zinc oxide dispersed in an electrically-insulating, film-forming resinous vehicle on one surface of said sheet and on the opposite surface of said sheet, a second photoconducting insulating coating comprising panchromatically-sensitive photoconducting zinc oxide dispersed in an electrically-insulating, filmforming resinous vehicle, said recording element thereby having two exposed photoconducting insulating surfaces.

l0. A method according to claim l wherein said first and second photoconducting layers have a backing member interposed between them, said first photoconducting layer and said backing being substantially translucent to said first light image, and wherein said first light image is projected through said first photoconducting layer incident upon said second photoconducting layer.

1l. A recording element having t-wo opposed layers, each having an exposed outer surface, one of said layers comprising a finely-divided photoconductive zinc oxide dispersed in an electrically-insulating, film-forming resinous vehicle, said first photoconducting layer being sensitive to and absorptive of light in a first spectral range and capable of being electrostatically-charged in, one polarity of charge, the other of said layers comprising a finely-divided photoconductive Zinc oxide dispersed in an electrically-insulating, film-forming resinous vehicle, said second photoconducting layer being sensitive to and absorptive of light in a second spectral range at least a portion of which is different from said first spectral range and said second photoconductive layer being capable of being electrostatically-charged in the op. posite polarity of charge, said first photoconducting layer being substantially translucent at least to light falling within said portion of said second spectral range.

12. A recording element for electrostatic printing having two exposed photoconducting insulating surfaces and comprising a backing sheet, a first photoconducting insulating coating on one surface of said backing sheet comprising a finely-divided photoconductive zinc oxide dispersed in an electrically-insulating, film-forming resinous vehicle, said first photoconducting coating being sensitive to light in a first spectral range and capable of being electrostatically charged in a first polarity of charge, a second photoconducting insulating coating on the opposite surface of said backing sheet comprising a different finely-divided Zinc oxide dispersed in an electrically-insulating, film-forming resinous vehicle, said second photoconducting coatingbeing sensitive to light in a second spectral range at -least a portion of which is different from said rst spectral range and capable of being electrostatically charged in the opposite polarity of charge, said backing sheet and said first photoconducting coating being substantially translucent at least to light falling within said portion of said second spectral range.

13. The recording element of claim l12 wherein said backing sheet is paper. y

14. The recording element of claim 9 wherein said backing sheet is paper and each said film-*forming vehicle comprises a silicone resin.

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Young et al.: RCA Review, vol. XXV, #4, pp. 469- 484 (1954). 20 

1. IN A METHOD OF ELECTROSTATIC PRINTING, THE STEPS COMPRISING: PROVIDING A RECORDING ELEMENT HAVING TWO OPPOSED LAYERS, EACH OF WHICH HAS AN EXPOSED OUTER SURFACE, ONE OF SAID LAYERS COMPRISING A FIRST PHOTOCONDUCTING INSULATING LAYER SENSITIVE TO AND ABSORPTIVE OF LIGHT IN A FIRST SPECTRAL RANGE AND CONSISTING ESSENTIALLY OF A FIRST FINELY-DIVIDED PHOTOCONDUCTOR DISPERSED IN AN ELECTRICALLY-INSULATING FILM-FORMING RESINOUS VEHICLE AND HAVING THE CAPABILITY OF BEING ELECTROSTATICALLY-CHARGED IN A FIRST POLARITY OF CHARGE, THE OTHER OF SAID LAYERS COMPRISING A SECOND PHOTOCONDUCTING INSULATING LAYER SENSITIVE TO AND ABSORPTIVE OF LIGHT IN A SECOND SPECTRAL RANGE AT LEAST A PORTION OF WHICH IS DIFFERENT FROM SAID FIRST SPECTRAL RANGE, SAID SECOND LAYER CONSISTING ESSENTIALLY OF A SECOND FINELY-DIVIDED PHOTOCONDUCTOR DISPERSED IN AN ELECTRICALLY-INSULATING FILM-FORMING RESINOUS VEHICLE AND HAVING THE CAPABILITY OF BEING ELECTROSTATICALLY CHARGED IN A SECOND POLARITY OF CHARGE, PRODUCING BLANKET ELECTROSTATIC CHARGE IN SAID FIRST POLARITY ON THE EXPOSED SURFACE OF SAID FIRST PHOTOCONDUCTING LAYER AND PRODUCING A BLANKET ELECTROSTATIC CHARGE IN SAID SECOND POLARITY ON THE EXPOSED SURFACE OF SAID SECOND PHOTOCONDUCTING LAYER BY SIMULTANEOUSLY SUBJECTING EACH OF 